Several studies have demonstrated that rotated segments of the embryonic hindbrain will repolarize to conform with the host gradients, and that the developing Mauthner (M-) s o n s reorient accordingly, tending to follow a normal course. To investigate the response of these axons to an irreversibly antagonistic polarity, we prepared Janus-headed telobionts in which initially descending M-fibers would begin to ascend upon entering the opposite head. About one-third of the fibers became disoriented within the graft region, evidently as a result of inadequate healing. The remaining two-thirds traversed the graft and continued to ascend the opposite head past the medulla and into the midbrain and forebrain. The apparent ease with which these M-axons made their way upgradient may have been enhanced by (1) inability of growth cones to respond to head-on gradients; (2) selective fasciculation; (3) weakening of the rostrocaudal (longitudinal) gradients as a result of birostral telobiosis; and (4) growth through intact tissue.
Most of these M-axons negotiated a rostrocaudal polar inversion in much the same way as commissural fibers normally do in passing from lateral to medial, then from medial to lateral. We suggest that crossing fibers may be impeded by their encounter with the contralateral antagonistic gradient, and that for this reason the rostrocaudal directional tendencies of decussating axons are expressed only after the midline has been crossed. This hypothesis does not require assumptions of right-left specificity or intricately coordinated timing of developmental events to explain decussation.